1. Field of the Invention
The present invention relates to improvements in an electric double layer capacitor, particularly in activated carbon which forms activated carbon electrodes included in polarizable electrodes.
2. Description of the Background Art
Generally, an electric double layer capacitor has an electric double layer formed at the surface of separation or interface between activated carbon electrodes and electrolyte included in polarizable electrodes, and electric charge is stored in said electric double layer. The activated carbon used for activated carbon electrodes is produced by carbonizing and activating vegetable materials (wood, wood flour, coconut shell, lignin, pulp waste water, etc.), mineral materials (coal, coke, pitch, etc.) or synthetic resins (phenol, polyacrylonitrile, cellulose, vinyl chloride, etc.).
In said electric double layer capacitor using activated carbon electrodes made of activated carbon obtained from vegetable or mineral materials, since activated carbon contains a large amount of ash, there is a drawback that the voltage retaining characteristic is poor.
Activated carbon obtained from synthetic resins contains almost no ash. In this case, in producing activated carbon the ease with which the pyrolyzed gas and activating gas diffuse must be taken into consideration, and in this respect it has been found that fibrous resins are suitable. Synthetic resins from which activated carbon is made are broadly classified into two types, one having no melting or softening point and the other exhibiting depolymerization and melting. Fibrous activated carbon from the former type is made from thermosetting resins, such as phenol resin, while fibrous activated carbon from the latter type is made from polyacrylonitrile and cellulose. When the two types are compared, it is seen that the polyacrylonitrile and cellulose series of the latter type are superior.
In the latter, it is necessary to effect flame resisting treatment by heat treatment in an oxidized atmosphere at 200.degree.-400.degree. C. prior to carbonization, and in order to reduce the oxidizing time involved therein, it is common practice to use compounds containing the group IV elements in the periodic table, particularly TiO.sub.2 (for example, Japanese Patent Application Laid-Open Specification No. 104910/1980). However, in that case, TiO.sub.2 remains in the activated carbon. If more than a certain amount of TiO.sub.2 remains in the activated carbon, Ti takes part in electrolysis, resulting in unignorable reaction current, which deteriorates the voltage retaining characteristic of the electric double layer capacitor.
In addition, the flame resisting treatment, which is a preparatory processing step in the production of fibrous activated carbon, is sometimes called infusibility treatment, thermal stabilization treatment or oxidation treatment.